The Green Monster

Epidote

Green Monster Mountain and the mine of the same name on Prince of Wales Island, Alaska, have been called that since at least 1900. I’ve looked extensively but I can’t find a definitive origin for the name, so I’ll rely on logic: The Green Monster is one of the premier locations in the world for huge dark green epidote crystals.

Epidote is a fairly common calcium-aluminum-iron silicate. It frequently forms pretty sheafs and sprays of somewhat radial green crystal groups, and individual crystals are by no means rare. It’s the large size at the Green Monster Mine that gets collectors’ attention, and sharp lustrous crystals on the order of 3 to 6 cm in length are fairly common. I think, but cannot verify, that the Green Monster name is from these huge green crystals.

My specimen here is so dark green that it is almost black, and it takes a microscopic look at internal reflections and thin edges to see the typical pistachio-green color of epidote. It is 26 x 21 x 17 mm, and I don’t absolutely know that it is from the Green Monster, because the label when I acquired it 50 years ago just said “Alaska,” but it’s pretty likely to be from there.

Epidote is a really common component of skarns, which are rocks that are both metamorphosed (changed under heat and pressure) and metasomatized (altered by the introduction of new chemicals, typically from magmas intruding older country rocks). At the Green Monster and nearby Copper Mountain in the Jumbo Mining District, limestone and mud rocks of Cambrian to Ordovician age (about 470 to 510 million years old) were intruded by granodiorite about 103 million years ago when these rocks were being accreted (amalgamated) to the North American continent. The resulting skarns contain rich deposits of copper, gold, and silver. The Jumbo District produced 4.6 million kilograms of copper (more than 10 million pounds), together with 220,000 grams of gold (7,800 ounces) and 1.73 million grams of silver (45,000 ounces) (Nokleberg and others, 1994, Metallogeny and major mineral deposits of Alaska: GSA DNAG Vol. G-1, The Geology of Alaska, p. 887).

The Green Monster, east of the main Jumbo Mining District and Copper Mountain, was explored for copper, but it has been a specimen-producing mine for decades. Epidote gets its calcium from the limestone country rock (which is metamorphosed to marble), and the granodiorite intrusion provided the iron, aluminum, and silica.

This specimen is actually a twin crystal of epidote, which you can see in the contact with angular patterns on the top termination of the crystal. Such twins, two distinct crystals that share a common interface, are common in epidote. For those who are interested in crystallographic details, epidote’s most usual twinning zone is {100}, the monoclinic front pinacoid.

The map above by C.W. Wright was published as part of USGS Professional Paper 87 in 1915.

“Epidote” comes from Greek for "increase", in allusion to the crystallographic characteristic of one longer side at the base of the prism. That’s one of the more obscure mineral name origins I know about, but who am I to criticize Rene Just Haüy, the mineral pioneer who named epidote in 1801.  The word “skarn” is from the Swedish word “skarn” which means skarn. Its ultimate heritage is from a word meaning dung or filth, a reference to the fact that the silicate minerals comprised waste rock compared to ore.

Prince of Wales Island is part of the Alexander Terrane. “Terrane” in tectonics means a block of mostly crustal rock with a distinct history, different from adjacent terranes with which it may be amalgamated. The Alexander Terrane was probably mostly an island arc system, but possibly with small continental fragments and even bits of oceanic crust, together with piles of sediments and small reef complexes. It extends from the southeast part of the Alaska Range in a long narrow belt (now in places tectonically dismembered) about 900 km (560 miles) to the coast of west-central British Columbia (Nelson and others, 2012, North Coast Project: Tectonics and Metallogeny of the Alexander Terrane, and Cretaceous Sinistral Shearing of the Western Coast Belt: British Columbia Geological Survey, Geological Fieldwork 2011, Paper 2012-1).

The Alexander Terrane is among the oldest of the blocks that now comprise the panhandle of Alaska and adjacent British Columbia, with rocks as old as Late Proterozoic (more than 550 million years) but also including much younger (Triassic, about 230 million years) volcanic rocks and others. It was once in equatorial latitudes before it amalgamated at least partly with the Wrangellia Terrane by about 300 million years ago. The combined Alexander-Wrangellia Terrane approached western North America and accreted to it during late Jurassic to early Cretaceous time, about 100 to 150 million years ago (Dostal and others, 2013, Bokan Mountain peralkaline granitic complex, Alexander terrane (southeastern Alaska): evidence for Early Jurassic rifting prior to accretion with North America: Canadian Journal of Earth Sciences, 50(6): 678-691).